1. Field of the Invention
The present invention relates to a digital modulation method for use in radio communications.
2. Description of the Related Art
In a digital mobile radio communication method, transmission and reception between a base station apparatus and communication terminal apparatus is influenced by the radio wave propagation environment, and the radio wave propagation environment influences reception quality and reception sensitivity characteristics on the receiving side. In this regard, heretofore, a method relating to the pilot symbol signal point position when performing quasi-coherent detection has been presented in the document, Rayleigh Fading Compensation Method for 16QAM MODEM in Digital Land Mobile Radio Systems, SAMPEI, Transactions of the Institute of Electronics, Information and Communication Engineers B-II Vol. J-72-B-II No. 1 pp. 7-15 Jan. 1989 as a method of improving the reception sensitivity characteristics of a receiving apparatus by devising a pilot symbol signal point position. FIG. 1 shows the signal point arrangement of 16QAM symbols and pilot symbols in the in-phase—quadrature plane (IQ plane). In FIG. 1, reference code 3501 indicates a 16QAM signal point in the IQ plane, and a method is known whereby the signal point that has the greatest amplitude among 16QAM signal points is taken as a pilot signal such that a pilot symbol signal point is placed at one of reference code 3502, reference code 3503, reference code 3504, and reference code 3505, and quasi-coherent detection is performed.
However, with conventional pilot symbol arrangement, a signal point with the greatest signal point amplitude of signal points in one modulation method is taken as a pilot symbol signal point, but when the reception sensitivity of the receiving apparatus is considered, this point is not necessarily at the optimum position for a pilot symbol signal point. Also, increasing the transmission power of the transmitting apparatus to improve the reception sensitivity characteristics of the receiving apparatus, and increasing the maximum signal amplitude shown in FIG. 1, means increasing the transmission power for all symbols to be transmitted, and thus increasing the power consumption of the transmitting apparatus.